Presently it is known to oxidize FeSO.sub.4 to Fe.sub.2 (SO.sub.4).sub.3 by HNO.sub.3 (nitric acid) (for example U.S. Pat. No. 2,196,584). The reaction is described by the following simplified equation: EQU 6FeSO.sub.4 +3H.sub.2 SO.sub.4 +2HNO.sub.3 .fwdarw.3Fe.sub.2 (SO.sub.4).sub.3 +4H.sub.2 O+2NO.
The disadvantage of this process is that the released NO has to be taken out of the reactor, oxidized to NO.sub.2, polymerized to N.sub.2 O.sub.4 and absorbed as HNO.sub.3. The oxidation of NO to NO.sub.2 has to be performed at low temperatures (below 120.degree. C.) and is accompanied by the evolution of a substantial amount of heat, requiring large heat exchangers for the removal of this heat. As the reaction itself is relatively slow to reach completion a large reaction space is needed to provide sufficient residence time and the HNO.sub.3 recovery section adds complications, is bulky and expensive. Another problem is that it is practically impossible to recover 100% of NO.sub.x and the emissions are, due to the poisonous nature of both NO and NO.sub.2, environmentally unacceptable.
Another process employed provides for the oxidation of ferrous iron at elevated pH. In this process, it is known that the rate of the oxidation in acidic solutions is negligible for all practical purposes, however the reaction proceeds quite rapidly at pH=5 or higher. This approach has three main disadvantages:
(a) The SO.sub.4 ions coming with the FeSO.sub.4 have to be neutralized. PA1 (b) The products of oxidation (ferric hydroxide and/or oxides) have to be separated from the products of SO.sub.4 neutralization. This operation itself poses a difficult problem which is compounded by the requirements to dispose of the sulphate solution. PA1 (c) Ferric oxides have to be reacted with the full amount of H.sub.2 SO.sub.4 to form Fe.sub.2 (SO.sub.4).sub.3.
Oxidation in an acidic solution by utilizing a catalyst--charcoal has been performed, however the rate of oxidation increases with molar ratio C/Fe.sub.t and at technically feasible values of this ratio the rate of oxidation is quite low. Attempts to increase the rate by increasing the charcoal content introduces frothing problems and a prohibitive cost of charcoal.
Another large group of processes utilizes oxidation by oxygen (either from air or in elemental form) catalyzed by NO.sub.x dissolved in the oxidized solution in a form of FeSO.sub.4 *NO.
Typical examples are:
1) United Kingdom Patent No. 17,112, N. McCulloch, 1894,
2) U.S. Pat. No. 4,693,881, R. Miller, 1987,
3) Australian Patent No. 71,741, Y. Mikami, 1974,
4) Japanese Patent No. 49-31638, S. Takada, 1974,
5) Japanese Patent No. 61-286228, Nittetsu Mining KK.
These processes are characterized by low operating temperature (&lt;60.degree. C.) within the range of stability of FeSO.sub.4 *NO complex and the oxygen (or air) is generally bubbled through the reacting solution. Oxygen has to enter solution to react with the NO complex thus the rate of reaction is hindered by low solubility of oxygen and the time required for completion of oxidation is very long. Quoting Mikami: "2.5 l. of ferrous sulphate slurry were oxidized for 17 hours". (This is to compare with our oxidation rate of 2.25 l. completely oxidized in 20 minutes.) Miller in his simplest version releases the desorbed NO.sub.x with off-gas to atmosphere. Quoting: "none of the NO which reaches the top of the regeneration reactor is recovered".
The process is not environmentally safe as it discharges large quantities of off-gas containing 0.03-0.15% NO.
Miller tries to solve this problem by complicated systems for recovery of the desorbed NO including conversion to and recovery as HNO.sub.3.
This is to compare with our system where NO is permanently closed in the reactor gas space and only a small bleed-off stream (approximately 3 m.sup.3 /h) is, after removal of NO by feed slurry, leaves the process.
It is therefore an object of this invention to provide an improved process for the manufacture of ferric sulphate.
It is a further object of the invention to provide such process which is very efficient and environmentally safe.
Further and other objects of this invention will be realized by those skilled in the art from the following summary of invention and detailed description of the embodiments thereof.